Date of this Version
1981 Poultry Science 60:1-21
Selection for egg production and egg quality traits in five Leghorn strains was based on pedigreed strain-cross (TX) progeny performance. About 24,000 TX pullets from 400 sires and 2400 dams were tested at 20 to 40 locations each year from 1956 through 1969. The TX pullet performance averages for 16 traits were combined in a linear index to evaluate sires in male lines and dams in female lines. Dams in male lines and sires in female lines were evaluated on the basis of full-sibs' progeny. Selection emphasis was primarily on rate of lay, viability, and egg size and secondarily on age at first egg, body size, egg shell and internal quality, temperament, and fertility. Each pair of pedigree crosses (TX1 or TX2) of one male line with two female lines was represented in a commercial cross (CXI or CX2) of one male line with line-cross females. Generation lag from TX to CX equalled about 2 years of relaxed selection. Genetic changes for TX, CX, and control strain (C) were estimated as phenotypic deviations from environmental trends measured by repeating the same generation and age of C breeders in consecutive years.
Yearly genetic gains in economic index per bird from 1956 to 1969 in TX1, to 1968 in TX2, to 1967 in CXI and CX2, and to 1966 in C were $.16, .18, .08, .11, and .10, respectively. Corresponding environmental trends averaged $-.13, -.11, -.14, -.14, and -.13 per year. Average yearly genetic gains for TX pullets exceeded those for CX pullets in adult mortality (-1.8 vs. -.5%), rate of lay (.50 vs. .17%), total eggs (2.9 vs. .8 eggs), and egg weight (.18 vs. .11 g) but were similar for other traits. Yearly genetic gain for total eggs and rate of lay accelerated significantly in TX pullets but not in CX pullets, possibly because TX response was measured for selection applied 3 or 4 years later than that for CX response. Contemporary superiority of TX over CX pullets was large for economic index ($.49), adult mortality (-5%), total eggs (12), and rate of lay (2.4%). When adjusted for the 2-year generation lag of CX, remaining superiority of TX over CX from temporary effects of pedigree selected parents and of pure line instead of line-cross dams was reduced to $.15 in economic index, -1.5% in adult mortality, 5.7 in total eggs, and 1.3% in rate of lay.